The present invention relates generally to cold rolled steel strip from which is made the core of an electric motor, and more particularly to steel strip which imparts to the core a relatively low core loss and a comparatively high peak permeability.
An electric motor is composed of a stator surrounding a rotor. The stator is composed of wire made from a relatively high conductivity material, such as copper, wound on a core composed of steel. The steel core of an electric motor is made up of laminations fabricated from cold rolled steel strip, typically composed of a silicon-containing steel, and the steel laminations impart to the core properties known as core loss and peak permeability which affect the power loss in the motor. Core loss, as the name implies, reflects power loss in the core. Peak permeability reflects power loss in the winding around the core. Core loss is expressed as watts per pound (W/lb.) or watts per kilogram (W/kg.). Peak permeability is expressed as Gauss per Oersted (G/Oe). Permeability may also be described in terms of relative permeability in which case it is expressed without units although the numbers would be the same as the numbers for the corresponding peak permeability. Core loss and peak permeability are both measured for the magnetic induction at which the core is intended to operate. Magnetic induction is expressed as Tesla (T) or kiloGauss (kG). A typical magnetic induction is 1.5 T (15 kG).
Thus, core loss reflects the power loss due to the core at a given magnetic induction, e.g., 1.5 T (15 kG), and peak permeability reflects the magnetizing current in the material of the core at that given induction. The higher the peak permeability, the lower the magnetizing current needed to achieve a given induction. In addition the higher the peak permeability for a given induction, the lower the power loss in the winding. Winding loss plus core loss are both important factors which reduce the efficiency of the motor.
Core loss and peak permeability are inherent properties of the steel strip from which the core laminations are fabricated. Therefore, an aim in producing steel strip for use in making the core of an electric motor is to reduce the core loss and increase peak permeability of that steel strip, both of which factors increase the efficiency of the motor. Both of these factors are affected by the composition and heat treatment of the steel strip.
Moreover, for a steel having a given composition and heat treatment, core loss increases with an increase in the thickness of the strip rolled from that steel. Thus, comparisons of core loss should be made on steel strips having comparable thicknesses. For example, assuming a core loss of 5.70 W/kg (2.60 W/lb.) at a strip thickness of 0.018 inches (0.46 mm.), if there is then an increase in thickness of 0.001 inch (0.0254 mm.), the core loss will increase typically at an estimated rate of about 0.22 W/kg (0.10 W/lb.).